[XFS] Ensure the inode is joined in xfs_itruncate_finish

On success, we still need to join the inode to the current transaction in
xfs_itruncate_finish(). Fixes regression from error handling changes.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30845a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>